God’s people in the Bible steadfastly held to the absolute singularity of God. For them, when speaking of God, one equaled one, and that one was their Father. However, post-biblical Christian theology saw a departure from that uncompromising monotheism. Binitarian and Trinitarian theologies proposed a different understanding of God. Binitarianism is the belief that two distinct persons are the one God. Trinitarianism is the belief that three persons are one God.
A Different Christianity / A New “Expanded” God
In the centuries after the Bible was written, non-Jewish people from the Hellenistic world entered the church. They were from backgrounds with very different ideas from those of the Bible. For thousands of years they had worshipped many individuals as divinity. In their pantheons, there had even been families of deity.
The overarching message of the Bible is not merely that there is one God. Rather, it is the more definitive declaration that there is only one individual who is God. That individual is YHWH— the Father of us all. Without the heritage of centuries of devotion to only one as God, the new Gentile converts often found themselves confused. Lacking a good understanding of the Hebrew Scriptures regarding the Messiah, they were often befuddled by such things as the disciples of Jesus bowing before him and calling him “Lord.” Would that not mean that Christ was also God in addition to YHWH? They even proposed that God’s own spirit is another person of Deity from the Father. Yet contrary to all of this, it was the Father himself who said:
I am the LORD, and there is no one else; other than me there is no God. I strengthen you, though you do not know me, so that all may know; from the east and from the west, that there is no one other than me; I am the LORD and there is no one else (Isa. 45:5, 6).
What these new converts were doing eventually resulted in a reconfiguring of the biblical understanding of God’s spirit and his Messiah. By the end of the 4th century CE, they were recasting them as divine persons who together with the Father were inexplicably supposed to be one God. But, when the prophets of old spoke of God’s spirit working and moving in the earth, they never said that it was another person from the Father. Rather it was the spirit of the Father himself. And to the prophets, God’s Messiah was his ultimate anointed king: the “Lord Messiah,” but not the “LORD God.”
From the 4th century till now, Gentile Christians have wrestled with the contradictions of the notion that God is multiple persons. How can two or three persons who are fully God not be two or three Gods? Did not the Bible say there is only one? And does it not repeatedly say that the Messiah is a human being? Christians have strained to see their idea of a multi-person God in the Bible. They have struggled against its insistence that there is only one individual who is God. Ultimately they concluded that there are two or three persons who are fully God, but that they are mysteriously just one God. 
The new theory about God was such a mystery, however, that no one in the Bible ever heard of it. There is no instance in the entire Bible where anyone argues for a two or three person God. Of the Scriptures that say there is “one God,” not one of them says two or three persons are the one God. On the other hand, as we have seen, a great corpus of Scriptures actually tells us that only one individual is God. Moses said:
So know this today, and take to heart, that the LORD, he is God in heaven above and on the earth below; there is no one else (Deut. 4:39).
Later, King Solomon also affirms that God is one individual:
So that all the peoples of the earth may know that the LORD is God, and that there is no one else (1 Kings 8:60).
The people of the Bible speak often of God, his spirit, and his Messiah. However, they never meant multiple persons who are one God. That was a mistake that God’s prophets of old and the original Christians never made.
Touted as a mystery, the idea of multiple persons as one God is actually the clashing together of two hopelessly inharmonious ideas: that three persons are each fully God, but there is only one God. The complexity, contradiction and confusion which accompany this theory are not worthy of the true God of heaven and earth. Regrettably, the clarity, simplicity and beauty of faith in only one individual as God have been replaced by an ongoing theological fog which envelops Christians to this day. Good people have struggled to believe in God and Christ not because of the confounding idea of the Trinity, but in spite of it.
Gill, J. Dan (2016). The Development of the Trinity. In, The One: In Defense of God (pp. 92, 243-245). Nashville, TN: 21st Century Reformation Publishing.
 Oneness (Pentecostal) theology is a third view and represents a variation on the same theme. In Oneness, rather than persons, God is pictured as having multiple manifestations. It is often difficult to see, however, any real difference between multiple persons and the supposed manifestations.
 The most famous is the family of Zeus who was supposed to be the father of Aphrodite, Athena, Apollo and others. The Greeks pictured these deities as being various individuals who were the same substance. That is essentially the concept employed by Gentile Christians in their idea of three ‘‘persons’’ who are one substance, what Trinitarian professor William Placher calls ‘‘stuff ’’ (i.e. Godstuff). See William Placher, A History of Christian Theology —– An Introduction (Philadelphia: The Westminster Press, 1983), 75.
 Jesus has this same understanding of the spirit in mind in Matthew 10:20.
 Dale Tuggy tells us that the formula of ‘‘three persons as one God’’ was not a part of post-biblical Christian vocabulary until the 4th century CE. Dr. Tuggy indicates that ‘‘there is simply no mention of a tripersonal God before the latter half of the 300s.’’ He believes that the ‘‘trinity’’ of earlier times is ‘‘just a triad, the founding member of which is the one God’’ (personal correspondence, February 12, 2015). Also see Dale Tuggy’s “10 Steps Towards Getting Less Confused About the Trinity —- #8 —- trinity vs. Trinity.” http://trinities.org/blog/10-steps-towardsgetting-less-confused-about-the-trinity-8-trinity-vs-trinity/.